1. Field of the Invention
The present invention relates to an apparatus for measuring a rotationally symmetric aspheric surface.
2. Description Related to the Prior Art
Three-dimensional shape measurement using an optical probe has been known as a method for measuring a rotationally symmetric aspheric surface such as an aspheric lens. This method, however, requires a long measurement time per sample, for example, several hours.
On the other hand, a method for identifying a shape of a sample surface based on interference fringes has been known. In this method, spherical wave light is emitted to a rotationally symmetric aspheric surface of a sample. Light reflected from the sample surface is made to interfere with reference light to form the interference fringes. However, when a relative position between an interferometer and the sample surface is fixed, only the interference fringes corresponding to an extremely limited region on the sample surface is obtained.
The following methods have been known, which solve the above problem. In a method disclosed in Japanese Patent Laid-Open Publication No. 62-126305, a relative position between an interferometer and a sample surface is varied sequentially to generate interference fringes corresponding to each of the regions on the sample surface divided in a radial direction. The interference fringes generated at each of the relative positions are analyzed to obtain a shape of each of the regions on the sample surface. Then, the obtained shapes are put together to get the shape of the entire sample surface.
In a method disclosed in U.S. Pat. No. 6,956,657, a relative position between an interferometer and a sample surface is sequentially varied in a plane perpendicular to a measurement optical axis. Every time the relative position is changed, interference fringes, corresponding to a region on the sample surface, are enlarged enough to be analyzed and then an image of the interference fringes is taken. The interference fringe images are analyzed to obtain the shapes of the regions. Then the shapes of the regions are put together.
The methods disclosed in the Japanese Patent Laid-Open Publication No. 62-126305 and U.S. Pat. No. 6,956,657 carryout the surface measurement of a sample in a shorter measurement time than the three-dimensional shape measurement using the optical probe. These methods, however, require a mechanism for sequentially varying the relative position between the interferometer and the sample surface. As a result, a configuration of a surface measuring apparatus becomes complicated and upsized.